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A primitive placoderm sheds light on the origin of the jawed vertebrate face
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Journal Article
A primitive placoderm sheds light on the origin of the jawed vertebrate face
2014
Overview
Studies of the head of the very primitive jawed vertebrate
Romundina
show that it combines jawed vertebrate architecture with cranial and cerebral proportions resembling those of extant jawless vertebrates such as lampreys.
The jawless-to-jawed transition
The development of jawed vertebrates from jawless fishes required more than just the addition of jaw to a jawless head — it necessitated a thorough reorganization of the skull morphology. Extant jawed vertebrates are so different from jawless ones (lampreys and hagfishes) that it is hard to see how this might have come about. Studies on a very primitive jawed vertebrate — the fossil placoderm fish
Romundina
— provide a glimpse of how that transition could have occurred. Vincent Dupret
et al
. show that the head of
Romundina
combines jawed vertebrate architecture with cranial and cerebral proportions resembling those of extant jawless vertebrates such as lampreys and some fossil jawless fishes. This combination appears to be primitive for jawed vertebrates and differs fundamentally from the morphology of sharks, traditionally used as a model for a generalized jawed vertebrate.
Extant vertebrates form two clades, the jawless Cyclostomata (lampreys and hagfishes) and the jawed Gnathostomata (all other vertebrates), with contrasting facial architectures
1
,
2
. These arise during development from just a few key differences in the growth patterns of the cranial primordia: notably, the nasal sacs and hypophysis originate from a single placode in cyclostomes but from separate placodes in gnathostomes, and infraoptic ectomesenchyme migrates forward either side of the single placode in cyclostomes but between the placodes in gnathostomes
3
,
4
,
5
,
6
,
7
,
8
. Fossil stem gnathostomes preserve cranial anatomies rich in landmarks that provide proxies for developmental processes and allow the transition from jawless to jawed vertebrates to be broken down into evolutionary steps
7
,
9
,
10
,
11
,
12
. Here we use propagation phase contrast synchrotron microtomography to image the cranial anatomy of the primitive placoderm (jawed stem gnathostome)
Romundina
13
, and show that it combines jawed vertebrate architecture with cranial and cerebral proportions resembling those of cyclostomes and the galeaspid (jawless stem gnathostome)
Shuyu
11
. This combination seems to be primitive for jawed vertebrates, and suggests a decoupling between ectomesenchymal growth trajectory, ectomesenchymal proliferation, and cerebral shape change during the origin of gnathostomes.
Publisher
Nature Publishing Group UK,Nature Publishing Group
